Wednesday, January 30, 2013

Troxler's fading

I have long been fascinated with optical illusions. Recently I have started seeing them as great examples of how our minds are continuously deceiving us and our intuition is often very wrong. Take the optical illusion below, for example. Stare at the center of the image (blinking is okay, just keep focused on one point).

If the illusion worked for you the colored image disappeared. This illusion is an example of Troxler's fading, which was first described by Ignaz Paul Vital Troxler in his 1804 paper "On the disappearance of given objects from our visual field". The effect happens because any stimulus that is unchanging is eventually ignored by the neurons receiving the stimulus. An even cooler example of Troxler's fading is this illusion.


At first you see pink dots that are disappearing in order around the circle. Focus on the cross, though, and the pink dots will disappear. In their place you will see a green dot circling the cross. Look very closely, that green dot doesn't actually exist. I expect the reason you see a green dot is because green is the negative color of pink. When your brain begins ignoring the pink dot stimuli it will still need to process when the stimuli change. That's tough though, your brain is already ignoring the pink dot, so how can it interpret something disappearing that it was already ignoring?The solution is to interpret the lack of a pink dot as a green dot - the negative of pink.